Emacs can highlight the current region, using X Windows. But normally
it does not. Why not?

Highlighting the region doesn't work well ordinarily in Emacs, because
once you have set a mark, there is always a region (in that
buffer). And highlighting the region all the time would be a nuisance.

You can turn on region highlighting by enabling Transient Mark mode.
This is a more rigid mode of operation in which the region "lasts"
only temporarily, so you must set up a region for each command that uses
one. In Transient Mark mode, most of the time there is no region;
therefore, highlighting the region when it exists is convenient.

To enable Transient Mark mode, type M-x transient-mark-mode.
This command toggles the mode, so you can repeat the command to turn off
the mode.

Here are the details of Transient Mark mode:

To set the mark, type C-SPC (set-mark-command).
This makes the mark active; as you move point, you will see the region
highlighting grow and shrink.

The mouse commands for specifying the mark also make it active. So do
keyboard commands whose purpose is to specify a region, including
M-@, C-M-@, M-h, C-M-h, C-x C-p, and
C-x h.

When the mark is active, you can execute commands that operate on the
region, such as killing, indenting, or writing to a file.

Any change to the buffer, such as inserting or deleting a character,
deactivates the mark. This means any subsequent command that operates
on a region will get an error and refuse to operate. You can make the
region active again by typing C-x C-x.

Commands like M-> and C-s that "leave the mark behind" in
addition to some other primary purpose do not activate the new mark.
You can activate the new region by executing C-x C-x
(exchange-point-and-mark).

C-s when the mark is active does not alter the mark.

Quitting with C-g deactivates the mark.

Highlighting of the region uses the region face; you can
customize how the region is highlighted by changing this face.
See section Customizing Faces.

When multiple windows show the same buffer, they can have different
regions, because they can have different values of point (though they
all share one common mark position). Ordinarily, only the selected
window highlights its region (see section Multiple Windows). However, if the
variable highlight-nonselected-windows is non-nil, then
each window highlights its own region (provided that Transient Mark mode
is enabled and the window's buffer's mark is active).

When Transient Mark mode is not enabled, every command that sets the
mark also activates it, and nothing ever deactivates it.

If the variable mark-even-if-inactive is non-nil in
Transient Mark mode, then commands can use the mark and the region
even when it is inactive. Region highlighting appears and disappears
just as it normally does in Transient Mark mode, but the mark doesn't
really go away when the highlighting disappears.

Transient Mark mode is also sometimes known as "Zmacs mode"
because the Zmacs editor on the MIT Lisp Machine handled the mark in a
similar way.